This invention relates to an FM receiver and more particularly to an FM stereophonic receiver equipped with a noise pulse suppression device.
Noise due to automobile electric motor wipers, particularly noise pulses emanating from automobile ignitions cause trouble in an FM receiver. Such noise pulse is capable of suppression to a limited extent by a limiter circuit after intermediate frequency amplification. However, in practice, the noise pulse is not sufficiently reduced or eliminated by such a limiter circuit. Heretofore, in order to more sufficiently remove the noise pulses, a noise suppression circuit 10 as shown in FIG. 1 has been employed.
Referring to FIG. 1, an FM detected composite signal containing a 19 KHz pilot signal, which is the output of an FM detection circuit 1, is delivered to both a delay circuit 2 and a noise pulse detection circuit 3 comprising a high-pass filter. The afore-mentioned noise pulse is detected out from the composite signal by means of the high-pass filter and then the composite signal is delivered to a shaping circuit 4 in the timed relationship with the occurrence of the noise pulse. The shaping circuit 4 constitutes a monostable multivibrator, which produces control pulses for a predetermined duration. On the other hand, the composite signal passing through the delay circuit 2 is applied to a gate circuit 5 whose gating operation is controlled in response to the output pulses of the multivibrator (4).
The gate circuit 5 operates to prevent the composite signal from being delivered to the next stage during the presence of the output pulses of the multivibrator (4). For the time period when the output pulse of the multivibrator (4) is applied to the gate circuit 5, the output of a level hold circuit 6 is delivered to the next stage. Hence, the level hold circuit 6 functions to hold the composite signal at the level immediately before the cut-off operation of the gate circuit 5. The outputs of the gate circuit 5 and the level hold circuit 6 are delivered to a multiplex (MPX) demodulator circuit 8 where the composite signal is separated into signals for right and left channels.
FIGS. 2(a) through 2(c) are waveform diagrams for operational description of the circuit shown in FIG. 1. FIG. 2(a) is the FM detected composite signal containing the 19 KHz pilot signal. Assuming now that a noise pulse (not shown) is superimposed on the composite signal at a time interval T between t.sub.1 and t.sub.2, the multivibrator (4) operates in response to the output of high-pass filter (3) to produce inhibit pulses for the duration of T. The gate circuit 5 is then put in a cut-off condition for the duration of T, and therefore the FM detected composite signal is not delivered to the next stage. In this situation, the level hold circuit 6 holds the level of the composite signal at the time t.sub.1 and the same is transmitted to the MPX demodulator circuit 8. The signal as shown in FIG. 2(b) is applied to the MPX demodulator circuit 8. The signal shown in FIG. 2(b) is demodulated into a waveform as shown in FIG. 2(c) after low-pass filtering in the demodulator circuit 8.
As can be appreciated, although the noise pulse introduced at the time interval T is removed, unwanted noise still remains as a part of the audio signal. Particularly, in the case where the audio signal is minute relative to the 19 KHz pilot signal, unwanted noise proportional to the level of the pilot signal becomes notable as can be seen from FIG. 2(c).